1. Field of the Invention
This invention relates generally to an apparatus for detecting the defects of an object, and more particularly, to an apparatus for detecting the defects of a photomask or object having linear straight line features and nonlinear defects, by using coherent light and incoherent light.
2. Description of the Prior Art
In the past, the detection of the defects of an object such as a mask pattern of a photomask for integrated-circuits was carried out by the visual observation of a human operator looking through a microscope. While somewhat satisfactory, it was difficult to distinguish the difference between defects and a proper mask pattern, as the chip-size enlarges and as the pattern becomes fine. Under such considerations, the human operator can rapidly become fatigued and apt to make misdetections. This, in turn, results in a lowering of the accuracy of detection.
In order to solve the above-mentioned problem, some methods for automatic detection have been suggested. One such method utilizes the directional character of the pattern. Thus, a mask pattern with linear straight line features formed by a combination of straight line components, is illuminated by coherent light, and a spatial filter for cutting the linear straight line features of the mask pattern, is provided at the Fourier-transform plane. This method is known as Watkins' method, and is described in the "PROCEEDINGS OF IEEE," April, 1972, pages 407 - 449. Again, while somewhat satisfactory, the defects detected by this method do not include position-information indicating where the defect exists in the mask pattern. However, whether a detected defect is important, or can be neglected, depends upon the position of the defect.
Consequently, a need exists for an apparatus for detecting the defects of an object, such as a photomask, which can conveniently and automatically judge whether a detected defect is an important one or not.